Many modern engines, including compression ignition engines, utilize a common rail fuel system for supplying fuel to each individual cylinder of the engine. In a common rail fuel system, a common rail fuel pump takes in low pressure fuel and supplies high pressure fuel to a common rail. Fuel injectors associated with each individual cylinder are fluidly connected to the common rail via individual branch passages. Over the years, the industry has demanded ever higher injection pressures, and hence ever higher common rail fuel pressures. As these rail pressures have exceeded 200 MPa and quickly approach 300 MPa, new problems have emerged in common rail fuel systems.
Common rail fuel pumps typically include two or more pump assemblies in a pump housing that are driven by a rotating cam shaft that includes one or more cams, each having one or more lobes. The different pump assemblies are typically out of phase so that the common rail can receive intermittent doses of high pressure fuel throughout the engine cycle to compensate for intermittent fuel injection from individual fuel injectors around the same engine cycle. In one particular example, a common rail fuel pump might include a cam shaft mounted for rotation in a pump housing. Rotational motion of the cam is translated into reciprocating motion of pump pistons by way of two or more individual tappet assemblies. Each tappet assembly includes a tappet that carries an axle about which a roller is rotationally mounted. The roller maintains contact with the rotating cam, and causes a reciprocating motion with each passage of a cam lobe. In order to function properly over an extensive working life, the good lubrication must be maintained for the roller, or premature wear and potential failure of the pump can occur.
The present disclosure is directed toward one or more problems set forth above.